The present invention relates to an image reading apparatus that reads out images on documents.
Prevailing of the electronic filing system in the market entails more attention focused on the image reading apparatus as an input device. The image reading apparatus employed in the electronic filing system is demanded to read documents at a higher speed for improving work efficiency. For the further prevailing of electronic filing systems in offices, the image reading apparatus is demanded to be smaller in size and less expensive.
The image reading apparatus employs an image sensor to read images of documents. There are two types of image sensors, one is a CCD image sensor and the other one is a contact-type image sensor. Each has respective advantages and disadvantages. The CCD image sensor reads fast but costs expensive and is difficult to be reduced in size. On the other hand, the contact-type image sensor costs inexpensive and is easy to be reduced in size, but reads not so fast.
In this present situation, various ideas are proposed to realize an image sensor capable of fast reading, inexpensive cost and compact size. Among these ideas, the following method is effective to satisfy this object, i.e. the contact-type image sensor is driven block by block so that a higher reading speed is obtained.
FIG. 11(a) is a block diagram that illustrates a reader employing a contact-type image sensor in a conventional image reading apparatus. As shown in FIG. 11(a), four contact-type image sensors 201, 202, 203, and 204 are arrayed in series in the main scan direction. Each of the above image sensors has 1280 pixels arrayed linearly. These contact-type image sensors are driven independently to read the image data on one line of the document. The image data on each line of the document is divided into four sections for being read out by the image sensors. The respective outputs from each of the contact-type image sensors are converted to serial data by an analog switch 205 besides functioning as a shift register, and then written into a buffer 206. Before being read out from the buffer 206, the serial data are re-arrayed in a normal pixel order, then supplied to an image processing circuit such as A/D converter.
As such, a time for reading one line is shortened through dividing one line into four sections (in the above-case, read-out time is shortened to xc2xc.) However, delays in reading occur between the pixels, whereby the resultant image data as shown in FIG. 11(b) are not continuous between each image sensor.
In other words, the image data, which are obtained through the block-driven method as shown in FIG. 11(b), progress of being read out also in a sub-scan direction while the image data progress of being read out in the main scan direction, thereby to produce discontinuity between each image sensor. In the case of reading binary images such as letters and symbols, there is no practical problems; however, in the case of reading an image including some gray scale such as a photographic document, which is supposed to undergo a dither process and an error-diffusion process, practical problems occur. That is to say, a vertical line appears in the pixel-discontinued-space.
The present invention addresses this problem and aims to provide an image reading apparatus that can avoid producing the discontinuity of pixels between each contact-type image sensor when one line of a document is read out. The image reading apparatus of the present invention also embodies a compact size, inexpensive cost, and higher process speed.
A first image reading apparatus of the present invention comprises:
document feeding device operable to feed documents;
document transmission device operable to transmit the documents fed by the document feeding device; and
reading device operable to read out an image on the document transmitted by the document transmission device.
In this reading device, plural contact type image sensors are arrayed in series in the main scan direction, and yet, a reading direction of each adjacent image sensor is set in a reverse direction with each other. Each image sensor comprises plural photo-sensors arrayed linearly.
A second image reading apparatus of the present invention further comprises a buffer operable to store image data. The image data that have been read out by the contact type image sensors are once stored in this buffer, and the stored image data are read out in the order corresponding to the main scan direction so that each pixel data is re-arrayed.
These structures can avoid producing discontinuity of pixels between each image sensor when plural contact-type-image-sensors read one line of a document through dividing the one line into plural blocks. These structures realize quality readout as well as allow the image reading apparatus to be smaller, less expensive, and faster in process.